The use of natural gas as an energy source in lieu of other hydrocarbons such as oil and coal is becoming ever more prevalent in the U.S. economy, in light of the discovery of substantial new reserves and the development of improved methods of extraction. The resulting reduction in cost of natural gas, in conjunction with cyclically high and widely variable cost of crude oil, makes natural gas a compelling low-cost and reliable alternative.
Due to the increased interest in using ever-larger volumes of natural gas and the locations of many new natural gas sources distances great enough from existing pipeline and gathering system infrastructure to make pipeline transportation economically impractical due to cost, there is a recognized need for improved product deliver infrastructure. In addition, the ongoing transition of motor vehicles to natural gas fuel necessitates creative solutions for providing access along transportation corridors, many of which are remote from pipelines or in areas where accessing a close pipeline is impractical due to cost, the developed nature of potential access corridors, environmental considerations, and other factors.
One solution to transportation of large quantities of natural gas is liquefaction, many enhancements to which have been developed by the inventors herein. Liquefaction enables transport from pipelines or even directly from a wellhead by truck or rail to points of use in local markets, where the liquid natural gas may be vaporized into a distribution system or used as a higher value liquid product for vehicle fuel, power generation, or industrial processes.
U.S. patent application Ser. No. 12/603,948 discloses a compact natural gas liquefaction process and plant utilizing a source of natural gas for both a natural gas processing loop and a refrigerant loop and enabling substantially all incoming natural gas to exit the plant as liquefied natural gas, avoiding return of natural gas to the source. The incoming gas stream is brought into the plant and circulated through compression, pressure reduction, and heat exchangers, pulling off a product stream equal to the mass flow entering the plant. The recirculation gas is always replenished at the same rate as liquefied gas production. This approach requires the use of larger compressors and flow paths than might otherwise be desirable, due to the continual recirculation process. Further, use of the recirculating design may be constrained in some circumstances by gas composition.
While the process and plant as disclosed in the '948 application facilitates liquefaction of natural gas in situations where natural gas cannot be returned to its source, there are conditions where it is desirable to separate a process stream from a refrigeration path in a compact natural gas liquefaction process and plant. For example, it would be desirable in some instances to avoid mixing of a refrigerant path and a process stream to better perform their respective functions. Separation of the two can, to some degree, reduce complications associated with different gas compositions. By using separate process streams and refrigerant paths, the refrigerant gas may comprise a single component or mixture to meet refrigeration requirements and may comprise any of a variety of refrigerants known by those of ordinary skill in the art, without limitation of selection by the composition of the product stream.
To elaborate on the foregoing, in at least some situations, it would be desirable to be able utilize different material compositions and design parameters (e.g., temperatures, flow rates, pressures) in each of the refrigerant and natural gas flows, as doing so may reduce cooling complications associated with certain natural gas source material compositions and may enable the use of a wider variety of refrigerants. Such a natural gas liquefaction process and plant may also decrease operating costs and increase process and plant efficiencies relative to previous natural gas liquefaction technologies by facilitating the use of smaller equipment (e.g., compressors) and smaller process flow paths. In addition, it would be desirable to have a very efficient method of liquefying natural gas from stranded sources, where there is no opportunity for a tail gas stream.